The extensive data storage needs of modern computer systems require large capacity mass data storage devices. A common storage device is the rotating magnetic disk drive.
A disk drive typically contains one or more smooth, flat disks which are rigidly attached to a common spindle. The disks are stacked on the spindle parallel to each other and spaced apart so that they do not touch. The disks and spindle are rotated in unison at a constant speed by a spindle motor.
Each disk is formed of a solid disk-shaped base or substrate, having a hole in the middle for the spindle. The substrate is commonly aluminum, although glass, ceramic, plastic or other materials are possible. The substrate is coated with a thin layer of magnetizable material, and may additionally be coated with a protective layer.
Data is recorded on the surfaces of the disks in the magnetizable layer. To do this, minute magnetized patterns representing the data are formed in the magnetizable layer. The data patterns are usually arranged in circular concentric tracks. Each track is further divided into a number of sectors. Each sector thus forms an arc, all the sectors of a track completing a circle.
A moveable actuator positions a transducer head adjacent the data on the surface to read or write data. The actuator may be likened to the tone arm of a phonograph player, and the head to the playing needle.
There is one transducer head for each disk surface containing data. The transducer head is an aerodynamically shaped block of material (usually ceramic) on which is mounted a magnetic read/write transducer. The block, or slider, flies above the surface of the disk at an extremely small distance as the disk rotates. The close proximity to the disk surface is critical in enabling the transducer to read from or write to the data patterns in the magnetizable layer. Several different transducer designs are used, and in some cases the read transducer is separate from the write transducer.
The actuator usually pivots about an axis to position the head. It typically includes a solid block near the axis having comb-like arms extending toward the disk, a set of thin suspensions attached to the arms, and an electro-magnetic motor on the opposite side of the axis. The transducer heads are attached to the suspensions, one head for each suspension. The actuator motor rotates the actuator to position the head over a desired data track. Once the head is positioned over the track, the constant rotation of the disk will eventually bring the desired sector adjacent the head, and the data can then be read or written.
As computer systems have become more powerful, faster, and more reliable, there has been a corresponding increase in demand for improved storage devices. These desired improvements take several forms. It is desirable to increase data capacity, to increase the speed at which the drives operate, to reduce the electrical power consumed by the drives, and to increase the resilience of the drives in the presence of mechanical shock and other disturbances.
In particular, there is a demand to reduce the physical size of disk drives. To some degree, reduction in size may serve to further some of the above goals. But at the same time, reduced size of disk drives is desirable in and of itself. Reduced size makes it practical to include magnetic disk drives in a range of portable applications, such as laptop computers, mobile pagers, and "smart cards".
An example of size reduction is the application of the PCMCIA Type II standard to disk drives. This standard was originally intended for semiconductor plug-in devices. With improvements to miniaturization technology, it will be possible to construct disk drives conforming to the PCMCIA Type It standard.
In order to shrink tile size of disk drives, every component must be reduced in size as much as possible. Size reduction of a component can not always be accomplished by merely scaling down. This is true in particular of tile spindle motor which rotates the disk.
Disk drive spindle motors are typically brushless direct current (DC) motors. These motors have a stationary set of electromagnets (the stator), and a set of permanent magnets attached to the rotating part of the motor (the rotor). The electromagnets of the stator are arranged in a circle surrounding the motor shaft. Each electromagnet includes a core (usually made of iron) surrounded by a coil (or winding) of electric wires. A motor controller, which is a set of switches and logic circuits on one or more circuit chips, sends pulses of electric current through the different coils to energize the electromagnets. The electromagnets are energized in a sequential pattern travelling around the shaft, inducing the permanent magnets in the rotor to follow, thus imparting a rotational force to the rotor.
Conventional disk drive spindle motors have generally employed one of two physical designs. In the first design, sometimes called a "pancake" motor, the stator electromagnets and rotor magnets are positioned under the disk stack. In order to reduce the overall height of the disk drive, the motor is made as flat and elongated as possible, giving it the name "pancake". However, even with a flattened motor, the design inevitably adds something to the overall height of the drive. In the second design the stator electromagnets and rotor magnets are contained within the holes of the disks on the disk stack. This design is referred to as an "in-hub" motor. An in-hub motor permits the overall height of the disk drive to be reduced, but it requires that the holes in the disks be sufficiently large to accommodate the motor components. The larger the holes, the less area that is available on the surface of the disks for recording data.
When attempting to shrink the size of conventional disk drive designs, the size of the spindle motor becomes a severely limiting factor. A conventional pancake motor is undesirable due to the need to reduce disk drive height, particularly in the case of the PCMCIA Type II form factor. An in-hub motor would appear to be the preferred design. However, the rotational force (torque) that the motor is capable of generating is related to the distance from the permanent magnet to the axis of rotation. As tile in-hub motor components are squeezed into the small space available in tile hole of a smaller disk, it becomes difficult to generate the torque needed for proper operation.
It is possible to design motors for PCMCIA drives using conventional techniques, but such designs involve trade-offs with other goals. Torque can be increased by increasing the amount of electric current pulsed through the windings, but this increases the power consumption of the drive and requires larger electronic components for pulsing the windings. Another solution is to enlarge the holes in the disks to increase the distance from the rotor magnets to the axis, thereby increasing the torque, but this reduces the area of the disk available for storing data. Alternatively, the motor can be operated at a slower speed, thereby reducing the amount of torque required, but this causes the disk drive to access data more slowly. It is desirable to develop a more compact disk motor design which reduces the need for these design trade-offs.